Group 13 exchange and transborylation in catalysis

Catalysis is dominated by the use of rare and potentially toxic transition metals. The main group offers a potentially sustainable alternative for catalysis, due to the generally higher abundance and lower toxicity of these elements. Group 13 elements have a rich catalogue of stoichiometric addition reactions to unsaturated bonds but cannot undergo the redox chemistry which underpins transition-metal catalysis. Group 13 exchange reactions transfer one or more groups from one group 13 element to another, through σ-bond metathesis; where boron is both of the group 13 elements, this is termed transborylation. These redox-neutral processes are increasingly being used to render traditionally stoichiometric group 13-mediated processes catalytic and develop new catalytic processes, examples of which are the focus of this review.

Gellrich reported the bis(pentafluorophenyl)borane-catalysed dimerisation of allenes, using various boronates as the terminal reductant (Scheme 5) [68]. Experimental and computational studies suggested the reaction proceeded by hydroboration of the allene 14 by bis(pentafluorophenyl)borane to give an allylborane 15, which underwent allylation of a second equivalent of the allene 14, giving a boryl diene 16. A Cope rearrangement of the boryl diene 16 followed by transborylation gave the dienyl boronic ester 18 and regenerated the catalyst (Scheme 5).
Chang reported the alkoxide-promoted hydroboration of N-heteroarenes with HBpin, the first explicit example of a B-N/ B-H transborylation in catalysis (Scheme 6) [69]. Reactive intermediates were characterised and BH 3 was observed to be generated in situ by the decomposition of HBpin. The proposed catalytic cycle involved nucleophile-promoted decomposition of HBpin to various borohydride species 19, which reacted with the BH 3 -coordinated heterocycle 21. Hydride transfer from the BH 3 -amide 22 to HBpin regenerated the borohydride catalyst 19, and gave a neutral aminoborane 23, which then underwent B-N/B-H transborylation with HBpin to give the N-Bpin dihydropyridine 24 and BH 3 (Scheme 6).
The mechanism of stoichiometric indole reduction with Me 2 S·BH 3 was investigated by Fontaine, and applied to a catalytic variant using HBpin as the turnover reagent (Scheme 7) [70]. Computational analysis showed two plausible, cooperative catalytic cycles: 1) hydroboration of indole 25 with BH 3 to give a H 2 B-N-indoline 26, which then underwent B-N/B-H transborylation with HBpin to regenerate BH 3 and give the N-Bpin-indoline product 27; 2) two molecules of H 2 B-N-indo- Thomas and Gunanathan independently reported the borane-catalysed double hydroboration of nitriles using either Me 2 S·BH 3 or H-B-9-BBN, respectively, as the catalyst and HBpin as the turnover reagent (Scheme 9) [72,73]. Both reports proposed similar mechanisms for the Me 2 S·BH 3 -and H-B-9-BBN-catalysed pathways (Scheme 9), where two pathways were operative. The hydroboration of the nitrile 36 gave a borylimine 37, which underwent a second hydroboration with the borane catalyst to give a diborylamine 38. Fontaine reported that boric acid could be used as a precatalyst for the BH 3 -catalysed hydroboration of esters, lactones, and carbonates with HBpin under microwave irradiation (Scheme 13) [57]. When HBpin and boric acid were reacted together, BH 3 -coordinated HBpin and O(Bpin) 2 were detected by 11 B NMR spectroscopy. Supported by computational analysis and single-turnover experiments, the reaction was proposed to occur by hydroboration of the carbonyl compound 53 with  Thomas reported the borane-catalysed diastereo-and enantioselective allylation of ketones with allenes and HBpin to give diastereo-and enantioenriched allylic alcohols, after workup (Scheme 15) [78]. The mechanism was investigated by singleturnover experiments and isotopic labelling and proposed to proceed by hydroboration of the allene 62 by the borane catalyst (H-B-9-BBN or 10-phenyl-9-borabicyclo  Yamamoto reported the borane-catalysed hydroalumination of alkenes and allenes (Scheme 18) [80][81][82][83] in which the organoaluminium products were reacted in situ with various electrophiles to give formal hydrofunctionalisation products (Scheme 18) [80][81][82][83]. Although no mechanism was proposed, a rare B-C/Al-H exchange may be responsible for catalytic turnover.
boryl-alkene 80 which underwent selective protodemetallation with another molecule of alkyne 1 to give the alkenylboronic ester 3 and regenerate an alkynylaluminium species 78 (Scheme 19b). Thomas et al. proposed a different mechanism for the diisobutylaluminium hydride (DIBALH)-or Et 3 Al·DABCO-catalysed hydroboration of alkynes [86], whereby an aluminium hydride 81 underwent hydroalumination of the alkyne 1, followed by Al-C/B-H exchange with HBpin, to give the alkenylboronic ester 3 and regenerate the aluminium hydride 81 (Scheme 19c). Single-turnover experiments and a lack of observable H 2 production supported this hypothesis. It should also be noted that nucleophilic bases, including LiAlH 4 , promoted the decomposition of HBpin to BH 3 which can mediate hidden catalysis [56].
Thomas et al. reported the aluminium-catalysed hydroboration of alkenes, using HBpin and LiAlH 4 as the catalyst (Scheme 20) [94]. Through single-turnover experiments they suggested a mechanism similar to aluminium-catalysed alkyne hydroboration; hydroalumination of the alkene 4 by the alane catalyst 80, Al-C/B-H exchange with HBpin, to give the alkylboronic ester 6 and regenerate the alane catalyst 80. A hydr ide-mediated decomposition of HBpin and hidden catalysis were not ruled out, as the use of LiH or NaH in place of LiAlH 4 gave moderate yields of the hydroboration product, however, comparison of the rates of reaction showed the aluminium had an active catalytic role (Scheme 20) [56]. Shi et al. reported that triethylaluminium catalysed the hydroboration of alkenes, under similar conditions to those of Thomas et al. [91]. Other ligand frameworks and aluminate species have shown competence for aluminium-catalysed alkene hydroboration [92,95], with Panda reporting the only reaction which proceeded at room temperature reaction using [κ 2 -{Ph 2 P(Se)NCH 2 (C 5 H 4 N)}Al(CH 3 ) 2 ] as the catalyst [90].
Using an ambiphilic aluminium precatalyst, (Me 2 N)C 6 H 4 AlMe 2 , Thomas et al. were able to shut down hydroalumination by the alane and catalyse the C-H borylation of terminal alkynes with HBpin (Scheme 21) [96]. Through kinetic analysis, it was found that the rate of the alkynyl-Bpin product formation was fastest during catalyst activation, rather than during catalysis, leading to an in-depth investigation of catalyst activation using variable time normalisation analysis (VTNA) and kinetic isotope effects. A catalytic cycle was proposed in which (Me 2 N)C 6 H 4 AlH 2 83 underwent deprotonation A number of aluminium hydride species has been used for the catalytic hydroboration of imines [87,92,97], nitriles [92,[98][99][100][101], carbodiimides [92,100,102], pyridine [92], and isocyanides [92] with HBpin (Scheme 23). These generally follow a similar proposed catalytic cycle; aluminium-mediated reduction, followed by Al-N/B-H exchange with HBpin (Scheme 23).

Gallium catalysis
Pioneering studies by Woodward reported the enantioselective reduction of ketones using HBcat and a mixture of MTBH 2 / LiGaH 4 as the catalyst, achieving high yields (up to 96%) and enantioselectivities (up to 93% ee) (Scheme 24a) [111]. The reaction was proposed to proceed through the enantioselective reduction of the ketone 95 by gallium hydride 96, followed by Ga-O/B-H exchange with HBcat to give an enantioenriched alkoxy catechol borane 98, affording the alcohol after workup (Scheme 24a). The mechanism was later explored in more detail, and the scope expanded, suggesting the reaction proceeded in a similar manner to the Corey-Bakshi-Shibata (CBS) reduction [112], whereby the gallium complex acts as an ambiphilic species coordinated to a ketone, activating it towards reaction with pre-coordinated HBcat [103].
Aldrich expanded the use of gallium in reductive catalysis by showing that a NacNac-supported gallium hydride catalysed the hydroboration of CO 2 with HBpin to give MeOBpin and O(Bpin) 2 (Scheme 24b) [113]. Through single-turnover experiments, the gallium hydride was observed to reduce CO 2 giving a gallium formate complex, which underwent Ga-O/B-H exchange with HBpin to afford O-Bpin formate and regenerate the gallium hydride. The analogous NacNac-supported alumini-um complex was not catalytically competent for the hydroboration of CO 2 , which was rationalised by the unfavourable thermodynamics of the analogous Al-O/B-H exchange [114]. Hevia reported a combination of a tris(alkyl)gallium species and bulky N-heterocyclic carbene acted as an FLP for B-H insertion, and was used subsequently as a catalyst in the hydroboration of ketones, aldehydes, esters, and imines with HBpin [115]. Using an ONO-pincer-supported gallium hydride, Goicoechea showed the catalytic hydroboration of ketones and CO 2 with HBpin. This was also proposed to proceed by carbonyl reduction and Ga-O/B-H exchange (Scheme 24c) [116].
Schneider has shown that a mixture of Ga 0 , AgOTf, and 18-crown-6 catalysed the allylation of acetals, ketals, or aminals with allylic or allenylboronic esters (Scheme 25) [117]. The reaction was proposed to proceed by an activation of elemental gallium to a Ga I species [(18-crown-6)-Ga I ·(dioxane) n OTf] 99, which abstracted methoxide from the acetal 100, to give an oxocarbenium 101 and Ga I OMe 102. The gallium(I) methoxide (102) underwent Ga-O/B-C exchange with allyl-Bpin 103 to give MeOBpin and an allylic gallium(I) species 104, which reacted with the oxocarbenium 103 to give the allylic ether 105 and regenerate the Ga I catalyst 99 (Scheme 25). Using allenylBpin, the selective propargylation of acetals was also achieved. When AgOTf was replaced with silver (R)-BINOL phosphate, the asymmetric allylation proceeded in a moderate yield (60%) and enantioselectivity (40% ee). The structure of the 'Ga I OTf' species was explored in more detail by Slattery, and a monovalent [Ga I (18-crown-6)OTf] complex was isolated and characterised by X-ray crystallography, lending support to the mechanism proposed by Schneider [118].

Indium catalysis
Examples of group 13 exchange are limited with indium, even stoichiometrically [36,45], however Kobayashi demonstrated the In I -catalysed addition of allylic and allenylboranes to ketals, acetals, aminals, and alkyl ethers (Scheme 26) [119][120][121]. The proposed mechanism was analogous to the Ga I catalysis by Schneider, with an In-O/B-C exchange proposed to drive catalytic turnover.

Conclusion
Increasing concerns over the sustainability and toxicity of many transition-metal catalysts has led synthetic chemists to seek alternative elements for catalysis. Group 13 compounds have been at the forefront of chemical research for the past century as organic reagents and functional handles. Group 13 exchange reactions have enabled these reagents to move beyond stoichiometric reactivity to be rendered catalytic, and exhibit catalysis outwith Lewis acid-type activation. These exchange reactions have allowed redox-neutral catalysis complementary to and beyond the redox catalysis of the transition metals.
Boron, aluminium, gallium, and indium have all been demonstrated in catalytic transformations using group 13 exchange from alkene functionalisation to carbonyl reduction. The subtle differences in reactivity of the group 13 catalysts were used to enable unique catalytic reactivity and/or reaction chemo-or stereoselectivity, including cases where the stoichiometric reaction was rendered catalytic and, more significantly, where no stoichiometric precedent was known. Group 13 exchange reactions being the driver for new chemical reactivity and unique molecular disconnections. This is not to say that all stoichiometric group 13 reactions have been rendered catalytic, or all new reactivity discovered, leaving an exciting future for main group catalysis underpinned by group 13 exchange and transborylation reactions (Figure 1).